Heat transferer

ABSTRACT

A rotary air-to-air heat transferer, which is capable of transferring both sensible and latent heat from one air stream to another, includes a heat transfer media comprised of a latticelike support coated with a thin film of an aqueous solution of a hygroscopic material such as lithium chloride.

I Ulllted States Patent 1 1 1111 3,733,791 Dravnieks 14 1 May 22, 1973[54] HEAT TRANSFERER 3,176,446 4/1965 Siggelin ..ss/34 4 'l 4 [75]Inventor: Konstantins Dravnieks, Madison, 70'708 10/1969 we! at 55/3Primary Examiner-Charles N. Hart [73] Assignee: Wehr Corporation,Milwaukee, Ill. A -J h W, Michael et a]. [22] Filed: Aug. 13, 1971BSTRACT 57 [21] App]. No.: 171,500 I 1 A A rotary air-to-air heattransferer, which is capable of transferring both sensible and latentheat from one air (g1 Stream to another, includes a heat transfer media58 Field of Search ..55/34, 181, 390; prised of lattice'llke watedwith af film 62/94, 271; 165/7 of an aqueous solunon of a hygroscopic materlalsuch as lithium chlor1de. [56] Reerences (med 9 Claims, 3 DrawingFigures UNITED STATES PATENTS 2,792,071 5/1957 Pennington ..55/390PATENTEDWYZZIQB $733,791

SHEET 1 OF 2 arage 5 HEAT TRANSFERER BACKGROUND OF THE INVENTION Thisinvention relates to air conditioning systems, and more particularly, torotary air-to-air heat transferers therefor.

Rotary air-to-air heat transferers are well-known in the airconditioning art. They are used in air conditioning systems to remove acertain portion of the heat from one air stream, such as from theexhaust stream from a heating system, and transfer same to another airstream, such as the inlet stream to the system. By doing so, the totalenergy requirements for heating or cooling a building are reduced;hence, initial equipment and operating costs are also reduced. Theprimary component of such a heat transfer is a slowly rotating, opencylinder, commonly called a wheel, through which both the incoming andoutgoing air streams are passed. This wheel carries an air-permeablematerial as the heat transfer media.

Prior art heat transfer wheels generally fall into tw separatecategories insofar as the heat transfer mechanism by which they operateare concerned, (1) those which primarily transfer sensible heat from oneair stream to another and (2) those which primarily transfer moisture orlatent heat from one air stream to another.

Examples of wheels falling within the first category are disclosed inU.S. Pat. Nos. 2,563,415. As disclosed in these patents, the heattransfer media is a non-hygroscopic, air permeable, highlyheat-absorbent material, such as a metal wool. The metal wool extracts alarge portion of the sensible heat from one air stream and releases itto another air 2,464,766 and stream. Because of the negligible latentheat transfer,

the total heat transferred by these wheels is somewhat below thatpossible if both sensible and latent heat were transferred. Also, thesewheels are ineffective for controlling humidity.

An example of wheels falling within the second category is disclosed inU.S. Pat. No. 2,700,537. Wheels of this type, commonly referred to asmoisture transferers, utilize an air-permeable, liquid-absorbing,insulative material, such as excelsior, corrugated cellulose or asbestospaper, or similar filamentous material, impregnated with a hygroscopicliquid or salt solution as the heat transfer media. Since wheels of thistype transfer moisture from one air stream to another, they have thecapability of controlling the humidity of air being supplied by the airconditioning system, e.g., can act as a de-humidifier in a coolingsystem. However, these wheels generally provide poor sensible heattransfer. Since these wheels absorb moisture, systems in which they areemployed generally include an auxiliary means for regenerating thehygroscopic material, i.e., relive the hydroscopic material of theabsorbed moisture. For example, a heated stream of air flowing from aseparate heater/blower system or from a sensible heat transferer (withor without supplemental heating) located in the main system flow, suchas described in U.S. Pat. Nos. 2,700,537, 3,009,540 and 3,251,402, havebeen used for regeneration of the hygroscopic material. U.S. Pat. No.3,176,446 describes another approach for regeneration wherein the wheelis washed with a fresh hygroscopic solution to displace themoisture-laden coating of hydroscopic material. Hence, these systemsemploy considerable auxiliary equipment to obtain a total heat transfer.

SUMMARY OF THE INVENTION -It has been found that a single wheel can beused to obtain a good total heat transfer if a lattice-like support iscoated with an aqueous solution of a hygroscopic material.

A primary object of this invention is to provide a heat transfer mediafor air heat transfer units utilized in air conditioning systems, andmethod for making same, having the capability of improved total heattransfer.

Another object of this invention is to provide a rotary air-to-air heattransfer having an improved capability of transferring both sensible andlatent heat.

According to this invention, the wheel for a rotary air-to-air heattransferer is packed with a heat transfer media comprised of alattice-like support, which is permeable to the axial flow of air, witha thin film of an aqueous solution of a hygroscopic material coating thesupport. The support can be rigid or semi-rigid and can be made from anymaterial which is chemically inert with respect to the liquid film, hassufficient structural integrity to withstand the force of the air streamand is wettable by the hygroscopic material. Suitable materials includemetal, such as pure aluminum, silver, titanium and tantalum; porous,cellular ceramic materials, particularly alumina; graphite; glass;mineral wool and synthetic plastic materials (in the form of a knittedlattice, a porous body, random filament or the like), such aspolyurethane and styrene polymers. The support has a negligible effecton the heat transfer, so there are no significant limitations on theheat transfer characteristics of the material used therefor, other thanthe material must be capable of operating in the thermal environment ofthe conditioning system without detrimental deformation.

The interstices of the support lattice can be uniformly oriented, suchas a knitted material, or can be randomly oriented, such as a glass ormetal wool, a cellular plastic or ceramic material and the like or, furthermore, can have a flat or curved shape. When a knitted material or awool is used, the individual member can have a circular cross section,be ribbon-shaped or have any other configuration which provides a largesurface area for exposure of the film of deliquescent material to theair stream flowing therethrough. When porous materials are use, they arepreferably provided with tubular passages which are generally parallelto the axis of the air stream to minimize resistance and maximizesurface contact between the film and the air stream.

The solution used to coat the support is an aqueous solution of awater-soluble, hygroscopic material capable of absorbing moisture fromthe air and releasing same when exposed to the environment of the airstreams. Representative examples of suitable solutions include acids,such as sulfuric acid, glycols, aqueous solutions of deliquescent salts,such as lithium chloride, lithium bromide, magnesium chloride, calciumchloride, calcium bromide, zinc chloride, magnesium fluoride, aluminumfluoride, potassium acetate, potassium carbonate, potassiummetaphosphate, magnesium perchlorate, phosphorous pentoxide, tin andhafnium tetrachlorides, and solutions containing mixtures of thesesalts, The water used in making the solution should not contain anysubstances which might react with either the support material or thehygroscopic material, and is preferably de-ionized or distilled.

The concentration of the solution is not particularly critical. Ahydroscopic material solution is somewhat self-adjusting because itrapidly reaches an equilibrium within a wide range of temperatures andhumidity conditions by either absorbing moisture from or releasing waterto the air stream flowing in contact therewith.

The solution can be applied in any suitable manner capable of providinga thin film thereof on the support, such as by dipping, spraying and thelike. If desired, the support can be packed into the wheel and thesolution sprayed onto if after installation. In order to preventcontamination to the film and to insure good adhesion, the support ispreferably treated with a suitable cleaning solution to remove anymaterials which may adversely react with the film solution and/or makethe surface of the base material hydrophobic with respect to thesolution. When a metal is used as the base material, the metal should beof high purity in order to minimize contamination to the film solutionand also to reduce corrosion from substances in the air streams flowingin contact therewith.

In some cases, especially those where a synthetic material is used asthe base material, it may be advantageous to precoat the base materialwith an organic or inorganic wetting agent, such as polyvinyl alcohol,mineral flour and the like, to improve the wettability of the surface,and hence, the coverage of the film solution. Alternatively, soluablewetting agents and/or gelling agent capable of improving the adherenceof the film to the support can be added to the hygroscopic materialsolution.

BRIEF DESCRIPTION OF THE DRAWINGS I DESCRIPTION OF THE PREFERREDEMBODIMENT The detailed structure of a wheel assembly for a rotaryair-to-air heat transferer (other than the heat transfer media) is notan essential part of this invention; therefore, only a generaldescription of the same will be given. US. Pat. Nos. 2,563,415 and2,818,934 disclose exemplary wheels which can be used for thisinvention. Co-pending application Ser. No. 880,544, filed Nov. 28, 1969and assigned to the assignee of the present application, discloses awheel assembly which is particularly adaptable to this invention. Forthe sake of brevity and completeness of disclosure, reference is made tothe above-identified patents and co-pending application for details ofthe structural aspects of the wheel assembly and the associatedequipment.

Referring to FIGS. 1 and 2, reference numeral generally designates awheel assembly for a heat transferer embodying this invention. Wheelassembly 10 includes a cylindrical outer casing 12, a central hub 14 anda plurality of generally imperforate blades 16 extending outwardly fromhub 14 and dividing casing 12 into a plurality of pie-shapedcompartments 18. Each compartment 18 is filled with a heat transfermedia 20, e.g., pure knitted aluminum fabric which has been dipped in anaqueous lithium chloride solution. Casing l2, hub 14 and blades 16 aregenerally about the same width in the direction parallel to the axis ofrotation of wheel assembly 10. When a metal wool or similar material isused as the support lattice, heat transfer media 20 is packed intocompartments 18 loosely enough to be air-permeable but compactly enoughto beselfsustaining or somewhat immovable within compartments 18. Inorder to insure that the heat transfer I media is maintained in place, awire mesh screen 22 or the like can be secured to each face of casing12. When the support lattice is composed of a porous or cellularmaterial, which has passages oriented to permit axial flow of air andsufficient structural integrity to support its own weight, theimperforate blades are not required.

The support lattice heat transfer media 20 is configured to have a highsurface area exposable to air streams 32A and 34C flowing therethroughso as to maximize the heat transfer between the air streams and the filmof lithium chloride solution coating the support while creating aminimum pressure drop to the air. For example, a metal mesh havingcircular wires of about 0.008 inch diameter produces a low pressuredrop. A metal mesh made from a flat, ribbon-like wire having a thicknessof about 0.002, even though having a higher pressure drop than thecircular wire mesh, has a substantiaily greater surface area per unitmass and provides a higher heat transfer efficiency,

Wheel assembly 10 is rotatably mounted in a housing or duct 24 which isseparated into two air passages 26 and 28 by wall 30. As wheel assembly10, driven by an electrical motor and axle (not shown) or similarconventional motive means operatively connected to hub 14, slowlyrotates at about 4 to about 20 R.P.M., heat transfer media 20 isalternately exposed to the air streams in passages 26 and 28 which flowaxially therethrough. During this rotation, the heat transfer media 20extracts latent and sensible heat from one air stream and transfers sameto the other air stream. The transfer of heat between the heat transfermedia and the air streams as the wheel assembly rotates through acomplete cycle will be described for the purposes of illustrating theoperation of the invention.

In FIG. 1, arrows 32A, 32B, 34C and 34D represent a supply air streamentering a building from outdoors through air passage 26,-the samesupply air stream after passing through the wheel assembly, a return airstream being exhausted from a building through air passage 28 and thesame return air stream after passing through the wheel assembly,respectively. Wheel assembly 10 is filled with a knitted, high purityknitted aluminum wire mesh which was dipped in an aqueous lithiumchloride solution prior to installation. In FIG. 2, a localized area ofthe heat transfer media is shown in phantom and designated by referencenumeral 36. Various positions of this localized area while wheelassembly 10 is being rotated in the direction of arrows through the twoair streams are designated by reference numerals E, F, G and I-I. FIG. 3graphically illustrates the thermodynamic characteristics of the two airstreams flowing through thewheel assembly.

In a typical air conditioning system, the return air stream from thebuilding will be comparatively cool and dry, e.g., a temperature of74.5F. dry bulb and 61F. wet bulb with 47 percent relative humidity,while the supply air will be hot and have a high humidity ratio, e.g., atemperature of 95F. dry bulb and 75.5F. wet bulb. Since only the liquidfilm participates in the heat transfer, only transfer of heat betweenthe air and the lithium chloride solution on the support material willbe described. At position E, where heat transfer media 36 is first movedinto the incoming hot, humid air stream 32A, it is relatively cool andthe concentration of the lithium chloride solution is high. Air stream32A at this point is shown as point J in FIG. 3. Associated with thishigh humidity ratio of stream of 32A is a high vapor pressure. Sensibleheat is transferred from the warmer air to the cooler lithium chloridesolution thereby cooling the air and warming the solution. Moisture inthe air is absorbed or condensed into the solution because of a vaporpressure gradient to the solution. The air becomes drier because of thisloss of moisture and the solution becomes more diluted. The release oflatent heat of condensation contributes to the rise of temperature tothe solution.

By the time the heat transfer media 36 has reached position F, aconsiderable amount of sensible and latent heat has been transferredfrom air stream 32A to the lithium chloride solution. Outgoing airstream 32B now has a temperature of 79F. dry bulb and 65.5F. wet bulbwith 48 percent relative humidity as shown as point K in FIG. 3. Thelithium chloride solution, on the other hand, has become hotter and nowcontains moisture condensed from air stream 32A.

At position G, the hot lithium chloride solution containing condensedmoisture is exposed to cool, relatively dry air stream 34C which iscooler and has a vapor pressure lower than the solution vapor pressure.Air stream 34C at this point is shown as point L in FIG. 3. Sensibleheat is transferred from the hot solution to the cool air and the airbecomes warmer while the lithium chloride solution becomes cooler.Simultaneously, water evaporates from the solution because its vaporpressure is higher than the vapor pressure of the air. The lithiumchloride solution becomes more concentrated as water is evaporated.

At position H, heat transfer media 36 has lost substantially all theheat accumulated during condensation (when the air flows are balancedand the process is continuous) and the lithium chloride solution hasbeen cooled. Air stream 34D, to which the latent heat or moisture fromthe lithium chloride solution has been transferred, has a temperature of90F. dry bulb and 71F. wet bulb with 40 percent relative humidity, andis shown as point M in FIG. 3. The lithium chloride solution, afterloosing the condensed water latent heat, is now cooler and moreconcentrated with a lower vapor pressure. In other words, it hasconditions similar to those at the start of the cycle at position B. Asthe wheel rotates into position E, heat transfer media 36 is ready foranother cycle.

Referring to FIG. 3, the total available heat in the system described inthis example is represented by reference numeral 38 with the total heattransferred being represented by the reference numeral 40. The totallatent heat is represented by reference numeral 42 with the transferredlatent heat being represented by the reference numeral 44. Totalsensible heat is represented by reference numeral 46 with thetransferred sensible heat being represented by reference numeral 48. Itcan be seen that, with a heat transfer media according to thisinvention, i.e., a lattice-like support coated with a film of an aqueoussolution of a deliquescent salt, the total heat transfer is in excess ofpercent. Furthermore, the presence of the liquid film of deliquescentmaterial provides the heat media of this invention with the capabilityof being used for humidity control.

As will be appreciated by those skilled in the art, when the above heattransferer is used in a heating system during the winter months,moisture and associated latent heat and total heat will be transferredfrom air stream 34C to air stream 323 thereby retaining heat andmoisture (corresponding to the efficiency of the heat transferer) withinthe building.

I claim:

1. An air conditioning system comprising,

means defining two separate passages for air flow,

a single rotary air-to-air heat exchanger wheel,

means supporting said heat exchanger wheel for rotation sequentiallythrough said two passages,

said heat exchanger wheel comprising a cylindrical, open-ended outercasing,

a heat transfer medium filling said casing,

said heat transfer medium consisting essentially of a lattice-likesupport distributed throughout the interior area defined by said casingand having individual members arranged to permit flow of said airstreams in an axial direction thereth rough with a minimum resistance;and

a film provided as a coating on the individual members of saidlattice-like support throughout the entire extent of said individualmembers in said casing, said film characterized by being an aqueoussolution of a hygroscopic material which is capable of exchangingmoisture and sensible heat with said air streams, said support beingsubstantially chemically inert with respect to said solution and beingfurther constructed and arranged to provide high surface contact betweensaid film and said air streams,

whereby said film coated lattice-like support alone in said air streamsprovides both sensible and latent heat transfer from the air flowing inone passage to air flowing in the other passage.

2. The heat exchanger according to claim 1 wherein said solutioncontains a wetting agent to enhance to wettability of the surface ofsaid support.

3. The heat exchanger according to claim 2 wherein said support ismetallic, glass or plastic material with the interstices between saidindividual members being uniformly oriented.

4. The heat exchanger according to claim 3 wherein said individualmembers have a generally circular cross section.

5. The heat exchanger according to claim 3 wherein said individualmembers have a fiat, ribbon-like shape.

6. The heat exchanger according to claim 2 wherein said support is ametallic, glass or plastic material with the interstices between saidindividual members being randomly oriented.

7. The heat exchanger according to claim 6 wherein said individualmembers have a generally circular cross section.

8. The heat exchanger according to claim 6 wherein said individualmember has a flat, ribbon-like shape.

9. The heat exchanger according to claim 7 wherein said support is aknitted pure aluminum fabric and said aqueous solution is a deliquescentsalt solution of lithium chloride.

2. The heat exchanger according to claim 1 wherein said solutioncontains a wetting agent to enhance to wettability of the surface ofsaid support.
 3. The heat exchanger according to claim 2 wherein saidsupport is metallic, glass or plastic material with the intersticesbetween said individual members being uniformly oriented.
 4. The heatexchanger according to claim 3 wherein said individual members have agenerally circular cross section.
 5. The heat exchanger according toclaim 3 wherein said individual members have a flat, ribbon-like shape.6. The heat exchanger according to claim 2 wherein said support is ametallic, glass or plastic material with the interstices between saidindividual members being randomly oriented.
 7. The heat exchangeraccording to claim 6 wherein said individual members have a generallycircular cross section.
 8. The heat exchanger according to claim 6wherein said individual member has a flat, ribbon-like shape.
 9. Theheat exchanger according to claim 7 wherein said support is a knittedpure aluminum fabric and said aqueous solution is a deliquescent saltsolution of lithium chloride.